1. Field of the Invention
The present invention relates to a droplet ejecting apparatus and a current control method, and particularly to a droplet ejecting apparatus and current control method for ejecting a droplet from an ejecting component.
2. Description of the Related Art
There have conventionally been known inkjet recording devices (so-called inkjet printers) having an ink jet recording head which, by using an actuator structured by a piezoelectric element or the like, changes the volume of (expands or contracts) a pressure generating chamber in which ink is filled, and, due to the change in pressure at the interior of the chamber caused by this change in volume, causes an ink drop to be ejected from the distal end of a nozzle which is formed to communicate with the pressure generating chamber.
The trend toward increasing the printing speeds of inkjet recording devices has strengthened in recent years. To this end, inkjet recording heads have come to be used in which the inkjet recording head is made to be longer, the number of nozzles per inkjet recording head is increased, and the nozzles are disposed so as to be arrayed in the form of a matrix, thereby enabling image formation over a wide region in a shorter period of time.
Thus, when the inkjet recording head is lengthened further than the width of a recording sheet, the inkjet recording head is provided with many ejectors formed from the above-described pressure generating chambers, nozzles, and the like.
In the case where the piezoelectric element is pulse-driven, an extremely large instant current is required when starting the drive of the piezoelectric element. Since the piezoelectric elements are simultaneously driven in the inkjet recording head, the total current is increased excessively when the current for starting the drive of the piezoelectric element is large.
For example it is assumed that a piezoelectric element having an electrostatic capacity of 600 pF is pulse-driven by power supply voltage of 20V while the pulse has a rise time of 1 μs. In this case, assuming that instant maximum current is about 30 mA and the inkjet recording head has the 1024 ejectors, the total current becomes about 30 A, so that maximum power supply capacity of about 600 VA is instantaneously required.
Thus, in the conventional inkjet recording head, the challenge has been to decrease the instant passage of the current through the piezoelectric element when starting the drive of the piezoelectric element.
Conventionally the rise time during which the voltage is applied to the piezoelectric element in starting the drive is lengthened in order to solve the problem (see Japanese Patent No. 3104304). However, in consideration of the generation of the small-diameter droplets (discharge: generation start to charge: cutting tail portion), the rise time cannot be lengthened too long. Particularly, for an ejector in which the pressure chamber is compressed by the charge, the desired droplet cannot be generated when the rise time is excessively long.
It is also thought that the charge and discharge are performed while output impedance of a drive circuit is decreased stepwise (see Japanese Patent Application Laid-Open No. 2002-094364). However, spike current is generated in the switching, and there is a possibility that the instant passage of the maximum current through the piezoelectric element is, conversely, increased.